Backfill for retaining walls



April 26, 1938. R E DQNCAS'TER 2,115,009

BACKFILL FOR RETAINING WALLS Filed Aug. 22, 1956 A I .PZ/ICELL anaasizr Patented Apr. 26, 1938 UNITED stars BACKFILL FOR RETAINING WALLS Purcell Eli Doncaster, Fort William, Ontario,

'Ganada 7 Application August 22,

2 Claims.

This invention relates to backfills for retaining walls and like engineering structures.

There is almost no field of engineering in which there is less uniformity and less accurate knowledge of basic factors than in retaining structures. Structures, which on the basis of accepted theory of pressure and resistance should have failed, have proved stable, and others built strictly in accordance with accepted practice and apparently with liberal factors of safety have failed.

There are few, if any engineering structures in which factors of safety and liberal allowance for contingencies are used as generously as in retaining structures, and yet in general the amount of materials used in and the costs of the construction of these structures appear unnecessarily large in comparison to what is considered adequate in other engineering structures. One of the main difficulties is that, under present practice, there is no control whatever of the pressure exerted by the fill or backing materials on the retaining structure, and it is this difliculty which it is the object of the present invention to remedy.

According to the invention the backfill is made up partly of the usual filling or backing material and partly of relatively light hollow members placed between the retaining structure and the plane of rupture of the fill material. As these hollow members have a much lower weight per unit of volume than the ordinary filling material, they not only allow the point at which the maximum pressure of the fill is exerted on the retaining structure to be controlled, but they cause a reduction in the total outward pressure exerted. This permits the structure to be made smaller or of lighter materials than would otherwise be possible, and may permit the use of a different type of structure than would otherwise be possible, such as steel sheet piling instead of gravity walls or wooden sheet piling instead of steel. In addition, where piling is used in soft foundation soil, it permits shorter piles to be used. The hollow members also present the advantage of reducing the passive resistance required of the earth at the toe of the structure, which is of importance in the case of relatively soft or unresistant foundations. Finally, by reducing the amount of fill materials as to whose actions and pressures there is so little clear knowledge, they reduce the factor of uncertainty in connection with the structure.

The hollow members may be left free in the backfill or held in any desired position by special means, in which case they have some effect in reducing the tendency of the rest of the backfill to 1936, Serial No. 91,459

exert an outward pressure against the retaining structure.

The invention will be more fully understood by reference to the attached drawing, in which:-

Figure l is a perspective view of a backfill embodying the invention.

Figures 2, 3, 4 and 5 are views of various means for retaining the hollow members in position in the backfill.

In the drawing a retaining structure is indicated at l and the backfill at 2. The backfill is shown as being made up partly of the usual backfilling material whose plane of rupture is indicated at 3 and whose plane of repose is indicated at 4. Between the plane of rupture 3 and the inner surface of the wall I are placed one or more relatively light hollow members 5 which may be either tubular, as shown, or of any other crosssection and may be made of any material which.

is strong enough so that the members will not collapse under the pressure exerted on them. As examples of suitable materials may be mentioned; plain or corrugated light steel pipe, such for instance as used for culverts; concrete pipe; or

wood stave pipe, creosote treated or not according to water conditions.

These light hollow members not only reduce the total outward pressure exerted on the retaining structure but make it possible to lower the point of maximum pressure on the latter. Gravity walls as illustrated are always made heaviest and their cross section is greatest near their base, and in the case of sheet piling, in connection with which the invention has perhaps its most important application, this lowering permits a reduction in the size of the tie rod and tie rod anchorages, which are expensive units. The lowering of the point of maximum pressure is most marked in the cases where the hollow members rest on a stringer supported on piles, since the members and the piles directly support a considerable portion of the fill directly above the members.

The hollow members may in certain cases be provided with holes in their lower are. This may be useful in the case of quay walls and the like wherethere is water on one side which rises and falls, since the holes will allow the members to fill when the water rises and-empty easily when it falls, so that extraneous water pressures are avoided. Moreover, if the water table is high, the members will act to carry offthe water to weep holes in the structure or on the ends of the fill when the latter is open ended.

In Figure 1 the members are shown as being free in the backfill and this is the way in which they may usually be arranged. However, in some cases, such as when the fill is in a plastic condition when placed, it may be necessary or desirable to fix the members in position in the fill in such a way as to prevent its subsequent movement. Various ways in which this might be done are illustrated in Figures 2 to 5. When the members are so retained in position they tend somewhat to reduce the outward pressure exerted on the retaining structure by the rest of the backfill.

In Figure 2 a hollow member is shown as being supported on piles and held against outward movement by means of a tie member 6 fixed to a suitable anchorage 7. In Figure 3 a member is supported on a cradle bent'8 and in Figure 4 on a different cradle bent 9, more completely restricting its movement. In Figure 5 a member is 2. Retaining wall construction as defined in claim 1, in which the hollow members are metal pipes.

PURCELL ELI DONCASTER. 

